I really am sorry if this question has come up so many times that is just makes you want to slap yourself and me very hard
Basically, I was wondering if these subjects (Biological Sciences and Biochemistry) are similar enough to realistically be able to apply for both at different institutions without jeopardising my chances of getting an offer... I am aware that my personal statement would have to cover both bases so could I just refer to the 'biosciences'?
Thanks in advance, any advice is much appreciated.

I think it depends entirely on your ability to avoid being specific while making it seem you are genuinely interested. For instance, if you want to say you are interested in a specific field or module, it should be one that is relevant to both courses.

At many institutions you might be able to switch courses after the first year anyway, that's what all the courses can do within biosciences at my uni. Maybe ask the unis? I think a uni would think it's fine if you don't know which one is for you.

I think that in your PS you should indicate that you have a great general interest in biology (from molecular biology to ecology). Then when you go in to more specifics, talk about interests in areas of biochemistry that would also be covered by a general biology degree, such as molecular biology, metabolism, proteomics, etc.